Calcium aluminate cements are produced by reaction of a calcium-containing material, typically limestone, with an aluminum-containing material, such as bauxite, to produce calcium aluminate. A typical calcium aluminate cement contains multiple phases of calcium aluminate, of which the main active phase is the hexagonal phase monocalcium aluminate (CaAl2O4 or CaO.Al2O3).
It is a well-documented problem that this hexagonal monocalcium aluminate is a metastable phase, which, in the presence of moisture, undergoes complex hydration reactions over long timescales to form the stable cubic phase hydrogarnet (CaO3.Al2O3.6H2O). This process is known in the art as ‘conversion’. The stable hydrogarnet phase is of greater density than the monocalcium aluminate phase; conversion of a calcium aluminate cement therefore results in an increase in porosity, and consequently a decrease in strength over time. In extreme cases, structures including calcium aluminate cements can collapse after several years due to strength loss resulting from conversion.
The conversion process has been the subject of extensive study over several decades, but attempts to address fully the problems that it raises have so far proved unsuccessful.
One way of managing the problem of decreasing strength due to conversion is to allow for the expected strength drop when designing a structure. The strength drop that can be expected to result from conversion can be predicted by testing and comparing the strength of converted and as-cast cement or concrete samples. For example, by exposing samples to a temperature above 140° F. (60° C.) the conversion rate can be accelerated such that samples will typically be fully converted within 24 hours (that is, all of the monocalcium aluminate has transformed to hydrogarnet). The strength of these fully-converted samples can be compared to that of as-cast samples to determine the expected strength drop over the life of the structure. Armed with this knowledge, engineers can ensure that the concrete will be of sufficient strength for its structural application even once conversion has occurred over the life of the product. However, in practice, the strength drop due to conversion can be unpredictable, making this approach unsuitable for many applications.
Alternatively, the problem of strength reduction can be mitigated to some extent by taking steps to minimise conversion of the cement to the hydrogarnet phase, for example by using a low water to dry mix ratio to reduce the water available for the conversion reaction, and/or by using concrete having a high cement content. However, the high cement content means that the resulting concrete is costly in comparison to alternative concretes of similar strength. Use of calcium aluminate cements in this context is therefore restricted to niche applications where properties such as rapid strength development, high-temperature strength or good chemical resistance are required.
There have also been attempts to prevent the conversion of calcium aluminate cements entirely by adding pozzolanic materials. For example, “The hydration of mixtures of monocalcium aluminate and blastfurnace slag” by Edmonds et al., published in Cement and Concrete Research, issue 19, p. 779 in 1989, describes a method for minimising conversion of a calcium aluminate cement in which half of the dry cement mix is replaced with granulated blast furnace slag. It is thought that the presence of the granulated blast furnace slag suppresses conversion by encouraging formation of stratlingite instead of hydrogarnet. “The influence of pozzolanic materials on the mechanical stability of aluminous cement” by Collepardi et al., published in Cement and Concrete Research Volume 25, Issue 5, p.961 in 1995 describes using alternative pozzolanic materials for the same purpose. Silica fume was found to be effective in reducing conversion, but fly ash was found to be ineffective.
Addition of pozzolanic materials to prevent conversion has several drawbacks. Firstly, the use of such large amounts of granulated blast furnace slag or other pozzolanic material reduces the strength of the cement, and secondly, some hydrogarnet is still formed. The inventors of the present invention have also observed that such cements display poor performance, particularly in cold weather. For example, working times are short, meaning that users have insufficient time to work with the cement prior to setting.
Finally, attempts have also been made to address the problem by preventing the initial formation of the metastable monocalcium aluminate, and instead forming the stable hydrogarnet and stratlingite phases directly as the cement sets. U.S. Pat. No. 4,605,443 to MacDowell, granted in 1986, describes a cement formed by fusing limestone, alumina and silica to form a glass, and then quenching the glass to produce stable hydrogarnet and stratlingite phases without intermediate metastable phases. However, production of this cement requires costly high-temperature processing due to the high melting points of the starting materials, making it unsuitable for many applications.
Despite many decades of research into conversion of calcium aluminate cements, a high-strength calcium aluminate cement that can be produced easily, that displays sufficiently long working times, and that sustains its high strength over long periods of time is not currently available. It is therefore an object of the present invention to provide such a cement, in which the problems found in the prior art are mitigated.